1. Field of the Invention
The present invention relates to a projector; more specifically, relates to an innovatively designed prism assembly for a projector for reducing size of the projector and providing higher contrast.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is the schematic view of a prior art projector 100 with smaller size. As shown in FIG. 1, projector 100 has a light system 110, a lens assembly 120, a digital micro-mirror device (DMD) 130 and a prism assembly 140.
Light system 110 generates lights, which emit to prism assembly 140; after that, the lights are reflected to DMD 130, which will again reflect the lights. DMD 130 comprises a dust-proof cover and a plurality of micro mirrors M. Micro mirrors M of DMD 130 are used to reflect the lights from the light system 110 reflected through prism assembly. Each micro mirror M rotates along a rotating axis to the ON state SON (the solid lines of DMD 130 in FIG. 1) or to the OFF state SOFF (the broken lines of DMD 130 in FIG. 1), respectively, according to a control signal. More specifically, each micro mirror M is in the FLAT state SFLAT before receiving the control signal, and is paralleled to the dust-proof cover of DMD 130. As receiving the control signal for enabling, micro mirrors M rotate clockwise to an angle θS; as receiving the control signal for disabling, micro mirrors M rotate counterclockwise to an angle θS. Therefore, the included angle of micro mirrors M between the ON state SON and the OFF state SOFF is 2θS. In the ON state SON, micro mirrors M will reflect the incident light through prism assembly 140, then into lens assembly 120 so as to project the light onto the projection screen. In the OFF state SOFF, micro mirrors M will rotate to an included angle 2θS to reflect the incident light through the prism assembly 140 so that after the light passes through prism assembly 140, it will carry on in the direction away from the optical axis A2 of the lens assembly 120 instead of entering into lens assembly 120.
Prism assembly 140 comprises two prisms TA and TB, and a medium layer X. Prisms TA and TB are usually glass pillars; prism TA comprises three planes P1, P2, and P3; prism TB comprises three planes P4, P5, and P6. Medium layer X is usually air layer locating between the plane P2 of prism TA and the plane P4 of prism TB. Prisms TA and TB have a refractive index N1, medium layer X has a refractive index N2; N2 is smaller N1, which means compare to prisms TA and TB, medium layer X is a less dense medium. When the light emits into plane P2 of prism TA from the light system 110 and the incident angle is smaller than the total reflection angle of prism TA, the total reflection will occur on plane P2. Additionally, plane P3 is paralleled with DMD 130, plane P5 is paralleled with lens assembly 120 (i.e. P5 is perpendicular to the optical axis A2 of lens assembly 120). The included angles between plane P1 and plane P2 and between plane P2 and plane P3 respectively are β and α. The included angle between planes P6 and P5 is γ, which is an acute or right angle, i.e. the angle γ is smaller or equal to a right angle.
Light system 110 is usually a gas discharge lamp using elliptic lampshade to gather lights, which emit along an optical axis A1. In other words, light system 110 is a light source with focal length f/# (f−number), in which the optical axis A1 is about perpendicular to plane P1.
Please still refer to FIG. 1. The lights from the light system 110 move along optical axis A1 and pass through plane P1; after emitting to prism TA, the lights are totally reflected from plane P2 to the plane of DMD 130 (i.e. the dust-proof cover of DMD 130) though plane P3, and an included angle between the normal to the plane of DMD 130 and the incident light is θAOI. Next, micro mirrors M will again reflect the incident lights. When in the ON state SON, the lights reflected by micro mirrors M (the solid lines in FIG. 1) will pass through plane P3 and be refracted between planes P2 and P4, then emit out from plane P5 to lens assembly 120. When in the OFF state SOFF, the lights reflected by micro mirrors M (the broken lines in FIG. 1 ) will pass through plane P3 and be refracted between planes P2 and P4, then emit out from plane P5 in the direction away from the optical axis A2 of lens assembly 120 instead of entering into lens assembly 120.
Please refer to FIG. 2. FIG. 2 is the schematic view of prior art projector 100 with lower contrast when in the OFF state. The lights from the edge of light system 110 pass through plane P1; after emitting to prism TA, the lights are totally reflected from plane P2 through plane P3, to DMD 130. Since the light system 110 has focal length F, the direction of lights from the edge of light system 110 is different than that from the center. As in the ON state SON, the lights reflected by micro mirrors M (the solid lines in FIG. 2) will pass through plane P3 and be refracted between planes P2 and P4, then emit out from plane P5 to lens assembly 120. As in the OFF state SOFF, the lights reflected by micro mirrors M (the broken lines in FIG. 2) will pass through plane P3 and be refracted between planes P2 and P3, then emit out to plane P6. After being totally reflected from plane P6, the lights will again emit out from plane P5 to lens assembly 120, as shown in FIG. 2. Thus the contrast of projector 100 will be reduced.
Please refer to FIG. 3. FIG. 3 is the schematic view of prior art projector 200 with high contrast. In FIG. 3, except prism assembly 240, the remaining elements are identical to those of projector 100; the description related to such functions thus will not be stated herein.
Similarly, prism assembly 240 also comprises two prisms TA and TB and a medium layer X. The lights from light system 110 move along the optical axis A1 and pass through plane P1; after emitting to prism TA, the lights are totally reflected from plane P2 through plane P3 to DMD 130, and an included angle between the normal to the plane of DMD 130 and the incident light is θAOI. Next, the micro mirrors M will again reflect the incident lights. As in the ON state SON, the lights reflected by micro mirrors M (the solid lines in FIG. 3) will pass through plane P3 and be refracted between planes P2 and P4, then emit out from plane P5 to lens assembly 120. As in the OFF state SOFF, the lights reflected by micro mirrors M (the broken lines in FIG. 3) will pass through plane P3 and be refracted between planes P2 and P4, then emit out from plane P5 in the direction away from the optical axis A2 of lens assembly 120 instead of entering into lens assembly 120.
Please refer to FIG. 4. FIG. 4 is the schematic view of the prior art projector 200 with increased contrast when in the OFF state. Lights from the edge of the light system 110 pass through plane P1; after being emitted to prism TA, the lights are totally reflected from plane P2 through plane P3 to DMD 130. Since the light system 110 has focal length F, the direction of lights from the edge of light system 110 is different than that from the center. As in the ON state SON, the lights reflected by micro mirrors M (the solid lines in FIG. 4) will pass through plane P3 and be refracted between planes P2 and P4, then emit out from plane P3 to lens assembly 120. As in the OFF state SOFF, the lights reflected by micro mirrors (the broken lines in FIG. 4) will pass through plane P3 and be refracted between planes P2 and P4, then emit out from plane P5 instead of entering into lens assembly 120, as shown in FIG. 4. Thus the contrast of projector 200 may be increased. Nonetheless, compare to prism assembly 140, prism assembly 240 has greater size hence the size of projector 200 becomes larger, that made it inconvenient for users.
Therefore in the OFF state, prior art projector 100 is not able to keep all lights away from lens assembly 120 (i.e. there are still stray lights entering into lens assembly 120), which would result in low contrast or even light leakage in projector 100. It is necessary to improve an image quality as bad as it is. In the prior art projector 200, however, size of prism TB in prism assembly 240 is increased to enhance contrast; thus the size of projector 200 is increased and that has made it inconvenient for users.